20° Roof Pitch
A 20° roof rises 364 mm for every metre of horizontal run, which is 4.37 in 12 in imperial terms (closest standard pitch 4.5/12). Its roof area multiplier is 1.064, so every square metre of plan area becomes 1.064 m² of roof surface.
Rafter length for a 20° pitch
Multiply the horizontal run by 1.064. Add the overhang (horizontal overhang × 1.064) and deduct half the ridge board thickness.
| Building span | Run (½ span) | Common rafter | Ridge rise |
|---|---|---|---|
| 4 m | 2 m | 2.13 m | 0.73 m |
| 6 m | 3 m | 3.19 m | 1.09 m |
| 8 m | 4 m | 4.26 m | 1.46 m |
| 10 m | 5 m | 5.32 m | 1.82 m |
| 12 m | 6 m | 6.39 m | 2.18 m |
Gable roof, metric. Rafter length is measured along the slope from wall plate to ridge centre line, before eave overhang and ridge board deduction.
| Building span | Run (½ span) | Common rafter | Ridge rise |
|---|---|---|---|
| 12 ft | 6 ft | 6 ft 4½ in | 2 ft 2¼ in |
| 16 ft | 8 ft | 8 ft 6¼ in | 2 ft 11 in |
| 20 ft | 10 ft | 10 ft 7¾ in | 3 ft 7¾ in |
| 24 ft | 12 ft | 12 ft 9¼ in | 4 ft 4½ in |
| 28 ft | 14 ft | 14 ft 10¾ in | 5 ft 1¼ in |
| 32 ft | 16 ft | 17 ft 0¼ in | 5 ft 10 in |
| 36 ft | 18 ft | 19 ft 1¾ in | 6 ft 6½ in |
| 40 ft | 20 ft | 21 ft 3½ in | 7 ft 3¼ in |
Gable roof, imperial. Rafter length is measured along the slope from wall plate to ridge centre line, before eave overhang and ridge board deduction.
Roof area at 20°
Roof surface area = plan area × 1.064. Use the roof calculator for multi-section roofs, waste and material costs.
| Plan area | Roof surface | With 10% waste |
|---|---|---|
| 80 m² | 85.1 m² | 93.6 m² |
| 100 m² | 106.4 m² | 117.1 m² |
| 150 m² | 159.6 m² | 175.6 m² |
| 200 m² | 212.8 m² | 234.1 m² |
| 250 m² | 266.0 m² | 292.6 m² |
Plan area is the footprint measured to the outside of the eaves. Multiply by 1.064 for the sloped surface.
| Plan area | Roof surface | Squares (+10%) | Shingle bundles |
|---|---|---|---|
| 1,000 sq ft | 1,064 sq ft | 11.7 | 36 |
| 1,500 sq ft | 1,596 sq ft | 17.6 | 53 |
| 2,000 sq ft | 2,128 sq ft | 23.4 | 71 |
| 2,500 sq ft | 2,660 sq ft | 29.3 | 88 |
| 3,000 sq ft | 3,193 sq ft | 35.1 | 106 |
Plan area includes eave overhangs. Bundles assume 3 bundles per square (100 sq ft) with 10% waste; hips, valleys and dormers need more.
Roofing materials for a 20° pitch (Australia & UK)
Typical manufacturer minimums. Check the specific product, sheet thickness, rafter length and exposure before ordering.
| Roofing system | At this pitch | Notes |
|---|---|---|
| Concealed-fix steel deck (e.g. Lysaght Klip-Lok 700) | Meets the typical 1° minimum | 1° applies to thicker sheet; 0.42 BMT is generally 2°. |
| Trapezoidal steel sheet (e.g. Lysaght Trimdek) | Meets the typical 2° minimum | — |
| Corrugated steel sheet (e.g. Lysaght Custom Orb) | Meets the typical 5° minimum | — |
| Interlocking concrete or terracotta tiles | Meets the typical 15° minimum | Profile dependent; many UK interlocking tiles need 17.5°–22.5°. |
| Natural slate | Meets the typical 20° minimum | Typical for larger slates; smaller slates and exposed sites need steeper pitches. |
| Plain tiles (clay or concrete) | Below the typical 35° minimum | Some products allow 30°. |
| Flat-roof membranes (EPDM, single-ply, felt) | Usually reserved for flat and low-slope roofs | Designed for a finished fall of about 1:80 (0.7°) or more. |
Roofing materials for a 20° pitch (US & Canada)
Minimum slopes from the 2021 International Residential Code. Canadian codes and manufacturers use similar limits for shingles and metal. Minimum pitch by material →
| Roofing system | At this pitch | Code section |
|---|---|---|
| Asphalt shingles | Meets the 2/12 minimum | IRC R905.2.2 |
| Clay & concrete tile | Meets the 2½/12 minimum | IRC R905.3.2 |
| Metal roof shingles | Meets the 3/12 minimum | IRC R905.4.2 |
| Slate & slate-type shingles | Meets the 4/12 minimum | IRC R905.6.2 |
| Wood shingles & shakes | Meets the 3/12 minimum | IRC R905.7.2 / R905.8.2 |
| Standing-seam metal panels | Meets the ¼/12 minimum | IRC R905.10.2 |
| Lapped (exposed-fastener) metal panels | Meets the ½/12 minimum | IRC R905.10.2 |
| Mineral-surfaced roll roofing | Meets the 1/12 minimum | IRC R905.5.2 |
| Membranes (TPO, EPDM, modified bitumen, built-up) | Allowed, but normally used on low-slope roofs | IRC R905.9–R905.13 |
Is a 20° roof steep?
Moderate pitches suit nearly every residential roofing material, shed water and snow well, and keep labour costs low because the roof is still workable.
A 20° roof is walkable for experienced roofers, but footing gets less forgiving on dew, dust or loose granules. Plan fall protection before going up.
20° roof pitch FAQ
What is a 20 degree roof pitch as a ratio?
20° is a rise of 4.37 in 12, closest to a standard 4.5/12 pitch. In metric that is 364 mm of rise per metre of run, or a slope of 36.4%.
How long is a rafter on a 20° roof?
Divide the horizontal run by cos 20°, which is the same as multiplying it by 1.064. On an 8 m wide building with a gable roof (4 m run), the rafter is 4.26 m before the eaves overhang.
How high is the ridge on a 20° roof?
Ridge height above the wall plate is the run × tan 20° (0.364). For an 8 m span (4 m run) that is 1.46 m.
Which roofing materials suit a 20° pitch?
20° meets the typical minimum for concealed-fix steel deck, trapezoidal steel sheet, corrugated steel sheet, interlocking concrete or terracotta tiles and natural slate. It is below the usual minimum for plain tiles. Minimums vary by product, sheet thickness, rafter length and exposure, so confirm with the manufacturer and local regulations.
Can you walk on a 20° roof?
A 20° roof is walkable for experienced roofers, but footing gets less forgiving on dew, dust or loose granules. Plan fall protection before going up.